The invention relates to a vehicle brake system having a gas pressure accumulator, which comprises a housing, the interior of which is divided by metal bellows into a gas-filled gas chamber and a fluid chamber, to which via a feed line a fluid may be supplied under pressure and removed. In said case, disposed between the fluid chamber and the feed line is a valve arrangement, which closes when the pressure in the feed line drops below a minimum value and opens when the pressure exceeds the minimum value. The invention further relates to such a gas pressure accumulator.
The fluid chamber of such gas pressure accumulators is filled, during operation of the vehicle brake system, counter to the pressure in the gas chamber partially or completely with brake fluid, in order to store the latter.
From DE 39 01 261 A1 a pressure accumulator for hydraulic systems is known, which comprises a housing, the interior of which is subdivided by two metal bellows into a gas chamber and a fluid chamber, wherein the latter is connected to the hydraulic system by a valve actuated by metal bellows. In said case, a valve body is fastened by a retaining body to one of the metal bellows, which moves the valve body onto a valve seat when the maximum admissible quantity of fluid has been removed from the fluid chamber. The valve body therefore closes the fluid chamber. With said valve it is possible merely to ensure that the metal bellows are not damaged in the event of a further drop of pressure in the hydraulic system. DE 39 01 261 A1 does admittedly provide a so-called accumulator charging valve but its function and mode of operation are not explained in said printed publication.
Particularly high standards are demanded of vehicle brake systems with regard to the operability and reliability of the equipment.
The object of the invention is therefore to overcome the previously described drawbacks and design the vehicle brake system having a gas pressure accumulator in such a way that the gas pressure accumulator remains operable even in the event of failure of another device of the vehicle brake system (e.g. the accumulator charging valve described in DE 39 01 261 A1).
Solution According to the Invention
The object is achieved according to the invention by a vehicle brake system of the type described initially having a gas pressure accumulator, in which the valve arrangement closes when the pressure in the feed line exceeds a maximum value and opens when the pressure drops below the maximum value. The object is further achieved by such a gas pressure accumulator.
By virtue of the design according to the invention the pressure in the fluid chamber of the gas pressure accumulator is limited to a maximum value, with the result that the metal bellows themselves remain operable even in the event of extremely high pressure in the feed line. The valve arrangement in said case performs a dual function. It closes the fluid chamber when the pressure in the feed line is below minimum pressure or above maximum pressure and opens it when the pressure is between minimum and maximum pressure.
An advantageous development of the gas pressure accumulator provides that the metal bellows during supply and removal of the fluid executes a stroke motion, by means of which the valve arrangement is actuated. Thus, closing of the fluid chamber is linked directly to the motion of the metal bellows, with the result that a self-contained safety system is formed.
In a first advantageous refinement of the invention, the valve arrangement comprises a piston, which is provided on the metal bellows and may be displaced along an axis between two sealing seats, which are arranged axially spaced-apart inside a hollow cylinder provided on the housing. In an alternative advantageous refinement, the valve arrangement comprises a hollow cylinder, which is provided on the metal bellows, is directed along an axis, has two internally disposed, axially spaced-apart sealing seats and is disposed in an axially displaceable manner around a piston provided on the housing. In said refinements the dual function of the valve arrangement is realized in a particularly simple manner.
An advantageous development of the sealing seats provides that the latter comprise paraxial sealing surfaces. The piston may slide against said sealing surfaces during axial displacement of the piston and/or of the hollow cylinder. The piston in said case retains the sealing function. As a result of elasticity or thermal expansion the volume of the gas and the fluid may change. The housing or the valve arrangement may moreover deform. In said case, the volume of the gas chamber and fluid chamber is slightly altered. In the case of the braking equipment according to the invention, the metal bellows are displaceable along the sealing surfaces and hence may compensate the differential pressures, which arise, without being damaged.
There is advantageously connected to the hollow cylinder a coaxial mandrel, on which the piston is guided or which is guided in the piston. By said means a guided movement of the piston relative to the sealing seats is possible and, at the same time, a compact form of construction of the gas pressure accumulator is achieved.
According to a development, the metal bellows are substantially in the shape of a hollow cylinder and the piston as well as the hollow cylinder are disposed radially inside the metal bellows, with the result that a particularly compact form of construction is achieved.
An advantageous refinement provides that the stroke motion of the metal bellows is delimited by two end stops in order to select defined end positions for the movable components. In the end positions the valve arrangement is in both cases closed.
A seal or a sealing seat is advantageously formed on at least one end stop. On the end stop, therefore, a redundant seal is formed, which enables particularly good sealing. In a particularly advantageous manner the redundant seal is disposed on the end stop delimiting the normal position of the piston. Thus, the gas pressure accumulator is sealed particularly well when the pressure in the feed line is lower than the admissible minimum pressure. The pressure in the feed line, the so-called system pressure of the vehicle brake system, may drop below said minimum pressure, the so-called gas admission pressure, especially during extended stationary periods of the vehicle.
The gas pressure accumulator may alternatively be provided with a valve arrangement, which is provided with at least one redundant seal on an end stop but does not have the dual function described above. Given such a valve arrangement, the piston as closing element during a closing motion first contacts a first sealing seat and effects sealing there. Then the piston contacts a second sealing seat, which forms an end stop for the closing element, and effects redundant sealing there. The first sealing seat may correspond to one of the sealing surfaces described above.
To guarantee the necessary sealing of the valve arrangement, at least one seal is advantageously disposed on the piston and may effect sealing against at least one sealing seat.